Interspecific crosses were carried out between the three Lactuca species L. virosa, L. serriola and L. sativa to transfer resistance to the leaf aphid Nasonovia ribis nigri from L. virosa to the cultivated lettuce, L. satira. L. serriola was used as an intermediate parent between the other two species. Many irregularities were observed in the interspecific hybrids, ranging from premature dying of Fl plants resulting from a kind of bastard necrosis till complete male and female sterility and deviating microsporogenesis. Using in vitro culture and after several backcrosses male and female fertile plants were obtained with a L. sativa habit and with resistance to the leaf aphid.

The intrinsic rate of increase (rm‐values), mean relative growth rates and mortality of Nasonovia ribisnigri (Mosley) (Homoptera, Aphididae) on different lines of lettuce (Lactuca sativa L.) were determined. Near isogenic resistant and susceptible lines, plus their ancestors, were used. Bionomics of N. ribisnigri on the resistant lines (NrNr) with the dominant Nasonovia resistance gene (Nr‐gene) differed clearly from the susceptible lines (nrnr). Mortality was high, no larvae reached adulthood, and no reproduction nor honeydew production was seen on the resistant lines. Transfer of aphids to susceptible plants after a period of 2 days on the resistant lines showed no signs of intoxication of aphids. Apparently there is no feeding on the resistant lines. It is not clear whether the aphids can not reach the phloem or do not accept it on the resistant line.

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INDEX WORDSLactuca, Laetuca sativa, lettuce, leaf aphids, Nasonovia ribis nigri, inheritance of resistance. SUMMARYThe inheritance of resistance to Nasonovia rib& nigri in L. sativa was investigated. Parents and F1 and F2 populations from crosses between the susceptible cultivar Ravel and two resistant breeding lines were tested. In both breeding lines one dominant gene appeared responsible for resistance.

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